The invention relates to a method for removing mercury and other elemental metals from emissions from combustion devices.
Much work has been done to remove pollutants from emissions from coal fired furnaces. The focus of most of these efforts has been toward the removal of particulates, NOx and SOx from flue gas. Commercially available techniques for reducing nitrogen oxide emissions in furnace flue gases include low-NOx burners, overfire air, selective non-catalytic NOx reduction (SNCR), selective catalytic reduction (SCR), and reburning.
Reburning is a technique whereby a fraction of the total thermal input to the furnace is injected above the primary combustion zone to create a fuel rich zone. Hydrocarbon fuels such as coal, oil, or gas are more effective NOx reducers than non-carbon containing fuels such as hydrogen or non-hydrogen containing fuels such as carbon monoxide. Stoichiometry of about 0.90 (10% excess fuel) in the reburn zone is considered optimum for NOx control. Thus, it is apparent that the amount of reburn fuel required for effective NOx control is directly related to the stoichiometry of the primary combustion zone and, in particular, the amount of excess air therein. Under typical furnace conditions, a reburn fuel input of over 10% of the total fuel input to the furnace is usually sufficient to form a fuel-rich reburn zone. The reburn fuel is injected at high temperatures in order to promote reactions under the overall fuel-rich stoichiometry. Typical flue gas temperatures at the injection point are above 1700K (2600xc2x0 F.). Overfire air is introduced into the flue gases downstream of the fuel-rich reburn zone in order to complete combustion of any unburned hydrocarbons and carbon monoxide (CO) remaining in the flue gases leaving the fuel-rich reburn zone. In addition, it is also known that rapid and complete dispersion of the reburn fuel in the flue gases is beneficial. Thus, the injection of reburn fuel is frequently accompanied by the injection of a carrier fluid, such as recirculated flue gases, for the purpose of promoting mixing. To the extent that the recirculated flue gas contains oxygen, the amount of reburn fuel must be increased.
U.S. Pat. No. 5,443,805 teaches injection of an additive such as ammonia with a small amount of hydrocarbon, preferably methane or natural gas, into flue gases at a temperature in the range of about 1228K to 1422K (1750xc2x0 F. to 2100xc2x0 F.)., and preferably 1355K to 1338K (1800xc2x0 F. to 1950xc2x0 F.)., for reducing pollutants such as NOx therein. Hydrocarbon is injected for the purpose of enhancing the NOx reduction efficiency of the ammonia additive in the temperature range of about 978K to about 1422K (1300xc2x0 F. to 2100xc2x0 F.). There is a similar teaching in U.S. Pat. No. 6,258,336. That patent also teaches that other nitrogenous compounds such as amines, urea, cyanuric acid and mixtures thereof can be injected with a hydrocarbon fuel downstream of the primary combustion zone.
While the art has focused primarily on the removal of NOx and SOx from flue gas, there are also concerns about emissions of mercury and other elemental metals such as chromium, arsenic and lead from combustion devices. Mercury (Hg), the eightieth element, is an important pollutant. As a vapor it is a poison of the nervous system. It is the dire consequences of chronic mercury poisoning which gave birth to the term xe2x80x9cMad as a hatter.xe2x80x9d Hatters that used mercury to block the hats were exposed to toxic levels of mercury vapor. The tremors, shakes, stutters, and stammers common to mercury poisoning were endemic in the trade. Neither were astronomers, who frequently used telescopes which were floated on mercury, immune from this disease. It was at times fatal and has the characteristic of being cumulative over years of exposure, as the body""s nervous system has difficulty in purging this element. Most industrial uses of mercury today are carefully controlled. The biggest sources of environmental mercury are coal combustion and the combustion of municipal solid waste. Coal and especially municipal solid waste compositions may also result in emissions containing chromium, arsenic and lead.
Mercury vapor is a poison. At the levels common in the atmosphere the concentrations are usually safe. However, the mercury accumulates in lakes where it is further accumulated in fish. These fish, with organic mercury molecules in them, can be hazardous to individuals who eat them. Some states request that people eat fish from some lakes no more frequently that once a week. Often it is stated that pregnant women and small children should eat no such fish.
Several states and the United States Environmental Protection Agency will soon limit the emissions of mercury and possibly other elemental metals from combustion devices. A method for controlling emissions of mercury and other metals is needed. Some control is possible by using particulate collection devices. However, only very expensive baghouses (fabric filters) are efficient enough to reduce the mercury to levels that may be required and still it is possible for the elemental mercury vapor to escape as a gaseous vapor molecule.
Activated carbon and other fine particulates are used to absorb mercury. Special treatment of the activated carbon has been tested. Collection by the use of activated carbon is very expensive. So, it is seen that a new method of removing mercury from flue gas is needed.
Mercury is emitted in power plant flue gases because the elemental form has a relatively high vapor pressure at usual stack flue gas temperature conditions. As such, the elemental mercury is emitted as a vaporous gas, Hg(v), which is very difficult to separate or filter; whereas if the mercury is oxidized it is no longer an elemental vapor. Moreover, the oxidized form exhibits a much lower vapor pressure and tends to collect or adsorb into surfaces of flyash particles within the flue gas. Those flyash particles are largely collected before the stack gas escapes. We have found that mercury can be oxidized to mercury chloride in the presence of background chlorine gas or hydrochloric acid gas when ammonia or ammonia precursor""s are made available in the flue gas and when the temperature of the flue gas is in the range of 1005K down to 755K (1,350xc2x0 F. down to 900xc2x0 F.). When oxidized the mercury is absorbed by particulates in the flue gas and removed with the particulates.
Mercury does not oxidize to stable concentrations of mercury chloride at temperatures above 1005K (1,350xc2x0 F.); while at temperatures below 755K (900xc2x0 F.) the rate of oxidation effectively ceases. In this temperature range (1,005K down to 755K), the rate of oxidation is increased by free chlorine radical (Cl) concentration, which becomes very limited in the presence of free hydrogen (H) radical concentration. The presence of increased water (H2O) thus limits the concentration of free chlorine radical in this temperature range and thereby tends to increase elemental mercury emissions; whereas the presence of ammonia and CO tend to decrease the free hydrogen (H) concentration and thus improve the oxidation of mercury to mercury chloride by providing higher instantaneous levels of free radical Cl. The free hydrogen may also be decreased by the reaction or combustion of hydrocarbon vapors which also provide the CO concentrations thus limiting availability of reactive H radical concentrations.
We adjust ammonia concentrations, available in the temperature range of 1005K (1,350xc2x0 F.) down to 755K (900xc2x0 F.), to provide maximum oxidation of mercury to mercury chloride, in the presence of CO, hydrocarbons, and sometimes NO and varying amounts of water. In this way the emissions of elemental mercury can be essentially eliminated while at the same time ammonia injection can be used for Selective Non-Catalytic Reduction (SNCR) of nitric oxide. Alternatively, ammonia can be injected independently in the zone of the furnace where the flue gas is at temperatures in the range of 1005K (1,350xc2x0 F.) to 755K (900xc2x0 F.) for elemental mercury emission control. Ammonia injected at this temperature range may also compliment the operation of Selective Catalytic Reduction (SCR) or various hybrid NOx removal systems although this temperature injection range is higher than normally used for SCR injection. Ammonia injection used at this temperature range also causes the oxidation of other elemental metals such as arsenic and lead which both poison the Selective Catalytic Reduction catalyst and are themselves hazardous stack gas emissions.